Cisco Systems 3750E manual Link Aggregation Control Protocol, Lacp Modes, 37-7

Chapter 37 Configuring EtherChannels and Link-State Tracking

Understanding EtherChannels

PAgP sends and receives PAgP PDUs only from ports that are up and have PAgP enabled for the auto or desirable mode.

Link Aggregation Control Protocol

The LACP is defined in IEEE 802.3ad and enables Cisco switches to manage Ethernet channels between switches that conform to the IEEE 802.3ad protocol. LACP facilitates the automatic creation of EtherChannels by exchanging LACP packets between Ethernet ports.

By using LACP, the switch or switch stack learns the identity of partners capable of supporting LACP and the capabilities of each port. It then dynamically groups similarly configured ports into a single logical link (channel or aggregate port). Similarly configured ports are grouped based on hardware, administrative, and port parameter constraints. For example, LACP groups the ports with the same speed, duplex mode, native VLAN, VLAN range, and trunking status and type. After grouping the links into an EtherChannel, LACP adds the group to the spanning tree as a single switch port.

LACP Modes

Table 37-2shows the user-configurable EtherChannel LACP modes for the channel-groupinterface configuration command.

Table 37-2	EtherChannel LACP Modes
Mode	Description
active	Places a port into an active negotiating state in which the port starts negotiations with other
	ports by sending LACP packets.
passive	Places a port into a passive negotiating state in which the port responds to LACP packets
	that it receives, but does not start LACP packet negotiation. This setting minimizes the
	transmission of LACP packets.

Both the active and passive LACP modes enable ports to negotiate with partner ports to an EtherChannel based on criteria such as port speed and, for Layer 2 EtherChannels, trunking state and VLAN numbers.

Ports can form an EtherChannel when they are in different LACP modes as long as the modes are compatible. For example:

•A port in the active mode can form an EtherChannel with another port that is in the active or passive mode.

•A port in the passive mode cannot form an EtherChannel with another port that is also in the passive mode because neither port starts LACP negotiation.

LACP Interaction with Other Features

The DTP and the CDP send and receive packets over the physical ports in the EtherChannel. Trunk ports send and receive LACP PDUs on the lowest numbered VLAN.

Catalyst 3750-E and 3560-E Switch Software Configuration Guide

OL-9775-02 37-7